1. Field of the Invention
The present invention relates to a method for manufacturing a solid electrolytic capacitor, particularly a method for manufacturing a solid electrolytic capacitor with an increased capacitance and a decreased ESR (equivalent series resistance).
2. Description of the Related Art
In recent years, along with digitalization and increase of frequency of electronic appliances, solid electrolytic capacitors are also required to having higher capacities than those of conventional ones and be excellent in impedance property in a high frequency region. This solid electrolytic capacitor 100, as shown in FIG. 1 and FIG. 2, is completed by following steps. Specifically, first, a capacitor element 10 is produced by the steps : forming a dielectric coating layer 2 by oxidizing the surface of an anode body 1 having a structure in which an anode lead 5 is implanted in a sintered body made of a valve metal (tantalum, niobium, titanium, aluminum, and the like), forming a solid electrolytic layer 3 (a first solid electrolytic layer 3a and a second solid electrolytic layer 3b) made of a conductive inorganic material such as manganese dioxide or a conductive organic material such as TCNQ complex salt, conductive polymer, or the like on the dielectric coating layer 2, and forming a cathode lead-out layer 4 composed of a carbon layer 4a and a silver paste layer 4b on the solid electrolytic layer. Next, the steps of connecting an anode terminal 7 to the anode lead 5 and connecting a cathode terminal 6 to the cathode lead-out layer 4 through a conductive adhesive 8 are performed and whereby, the capacitor element 10 is connected to the respective terminals 6 and 7. Furthermore a step of coating the capacitor element 10 with an external resin 9 such as an epoxy resin or the like is performed and whereby, the solid electrolytic capacitor is produced.
Herein, in the step of forming the solid electrolytic layer 3 composed of a conductive polymer, for example, proposed is a method for forming a relatively thick conductive polymer cathode layer by an electrolytic polymerization method using a water-based electrolytic polymerization solution on a thin conductive polymer cathode layer formed by a chemical polymerization method. (see, for example, Japanese Unexamined Patent Publication No.2002-289479)
More particularly, with respect to a solid electrolytic capacitor according to the above-mentioned conventional method, for example, a tantalum porous sintered body is used as an anode body and a dielectric coating layer of tantalum oxide is formed on the surface of the anode body. Further, a solid electrolytic layer is formed on the dielectric coating layer by successively forming a conductive polymer cathode layer by chemical polymerization and a conductive polymer cathode layer by electrolytic polymerization. The electrolytic polymerization conductive polymer cathode layer above is formed on the chemical polymerization conductive polymer cathode layer by immersing the anode body having the conductive polymer cathode layer formed by chemical polymerization in an electrolytic polymerization aqueous solution containing a monomer and an alkyl aromatic sulfonic acid salt bringing a metal wire for electric communication into contact with the conductive polymer cathode layer formed by the chemical polymerization, and carrying out electrolytic polymerization by applying electric current to the electrolytic polymerization solution.
However, according to investigations of the present inventors, a solidification capacity ratio, i.e., a ratio of the electrostatic capacitance by measured the anode body without immersion in an aqueous sulfuric acid solution with respect to the electrostatic capacitance (electrostatic capacitance in-water) by measured the anode body immersed in the aqueous sulfuric acid solution, is less than about 80%.
The reason why the low ratio is that, since the anode body of the porous sintered body such as tantalum has nanoporous structure, it is difficult to densely form the above-mentioned solid electrolytic layer in the insides of fine pores of the porous sintered body. Therefore, there is a problem that no sufficient electrostatic capacitance can be obtained.
Further, if constant current is continuously applied, the conductive polymer cathode layer formed by electrolytic polymerization is acceleratedly increased around the feeding point where the above-mentioned metal wire is brought into contact with the above-mentioned conductive polymer cathode layer formed by chemical polymerization. Consequently the distribution of the layer of the conductive polymer forming the solid electrolytic layer becomes uneven.
Further, since ESR (equivalent series resistance) of a solid electrolytic capacitor having such an uneven solid electrolytic layer tends to be increased, it is required to produce a solid electrolytic capacitor having lower ESR.